Biology Unit Three

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B3 - Cells

Animal Cell structures - The nucleus contains DNA in the form of chromosomes.                          The Cell Membrane holds the cell together and control what goes in and out.                               The Ribosome is where proteins are synthesised.                                                                         The Cytoplasm is a gel - like substance where most of the cells chemical reactions happen.             The Mitochondria is where most of the reactions involving respiration happen. Respiration provides energy. Liver and Muscle cells contain many mitochondria.

Plant Cell structures - The plant cells have all the features of animal cells and also have:     Chloroplasts - where photosynthesis happens                                                                                  A cell wall - to support the cell (made of cellulose)                                                                 Vacuole - a large structure which contains cell sap, a weak solution of sugar and salts.

Bacteria Cells - Bacteria cells dont have a nucleus. Instead they have a single loop of DNA which floats freely in the cytoplasm. They also dont have chloroplasts or mitochondria. 

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Chromosomes are made of DNA. Chromosomes are long molecules of coiled up DNA. The DNA is divided into short sections called genes.                                                                                      DNA is a double helix. Each of the two DNA strands is made up of small groups called 'nucleotides'. Each nucleotide contains a small molecule called a base. Each base forms a cross link to a base on the other strand.                                                                                                    A always pairs up with T and G always pairs up with G. 

Watson and Crick - Used data from other scientists to help them understand the structure of the molecule. This included x rays showing that DNA is a double helix formed from two chains wound together and other data that showed that bases occurred in pairs. They put this information together and made a model to show what DNA looked like.

DNA can replicate herself - DNA copies itself every time a cell divides so that each new cell has the full amount of DNA. In order to copy, the DNA first unzips to form two single strands. new nucleotides float freely in the nucleus and then join on using complimentary base paring . This makes an exact copy of the DNA on the other strand.

The result of this is two double stranded molecules of DNA which are identical to the original.

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Protein synthesis

DNA controls the production of protein in the cells - Genes code for particular proteins. Proteins are made of chains of amino acids. Each protein has its own number and order of amino acids. This gives each protein a different shape and therefore a different function. The order of bases in a gene decide the order of amino acids in a protein. Each amino acid is coded for by a sequence of three bases and are joined together to make proteins following the order of the bases in the gene.

mRNA carries the code to the ribosomes - Proteins are made in the cytoplasm by ribosomes. To make the proteins ribosomes use the code in the DNA. DNA is found in the nucleus however it can't move as its too big. mRNA is made by copying the code from the DNA. It can therefore act as a messanger and carry the code between the DNA and the ribosome.

DNA controls a cell by controlling protein production - Proteins affect the way a cell functions. Some determine cell structure, some control cell reactions. Different types have different functions because they make different proteins. Some genes are 'switched off' in the cell and therefore only some of the full sets of genes are available to make proteins. 

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Functions of proteins

Carrier molecules transport smaller molecules, eg: HAEMOGLOBIN BINDS TO OXYGEN.              Hormones carry messages around the body, eg insulin is produced in the pancreas to regulate blood sugar levels.                                                                                                                    Structure proteins are physically strong, eg, collagen strengthens connective tissues.

Enzymes control cell reactions - Reactions in the body have to be carefully controlled to make sure the right amount of substances are produced. Enzymes act as a biological catalysts. Catalysts speed up reactions without being changed by or used up in the reaction. Enzymes reduce the need for high temperatures and we only have enzymes to speed up useful chemical reactions in the body. Every biological reaction has its own enzyme designed especially for it, Each enzyme is coded for by a different gene and has a unique shape.

Enzymes are very specific - Chemical reactions involve things being split or joined. The substrate is the molecule changed in the reaction. Every enzyme has an active site where it joins on to the substrate and catalyses the reaction. Enzymes are picky: they have a high specifity for their substrate. For the enzyme to work, the substrate has to fit the active site. If the shapes dont match the reaction wont be catalysed. This is called the 'lock and key' mechanism.

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Enzymes like it warm but not too hot -A higher temperature increases the rate of reaction as the heat increases the amount of energy the enzymes and the substrates have meaning they collide with one another more. They have a higher collision rate. If the temperature is low, the opposite occurs. If it becomes too hot, the bonds holding the enzymes together break. Therefore the enzyme loses its shape and its active site won't fit the substrate anymore - the reaction cant be catalysed and the reaction stops. The enzyme has been denatured. Each enzyme has an optimum temperature, those in the human body's being 37 degrees.

pH - If the pH is too high or low it interferes with the bonds holding the enzyme together. Therefore the active site changes shape and the enzyme is denatured. All enzymes have an optimum pH - usually being pH7 (neutral), However some enzymes such as pepsin (breaks down proteins in the stomach) works best at pH2 (works best in acidic conditions).

Q10 values -shows how much the rate changes when the temperature is raised by 10 degrees celcius. Equation = Q10 = rate at higher temperature/rate at lower temperature.

A Q10 value of two would mean that the rate of reaction doubles when the temperature is raised by 10 degrees.

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Gene mutations - A mutation is a change in the DNA base sequence. If a mutation occurs within a gene it could stop the production of the protein the gene usually codes for - or it may force it to produce a different protein.

Most mutations are harmful -The wrong protein or no protein being produced can cause problems. If the mutation is in a reproductive cell the offspring may develop abnormalities. If a mutation occurs in body cells the mutant cells can start to multiply in an uncontrolled way and move around the body - this is what causes cancer.                                 

Beneficial mutations -If the newly produced protein is an improvement on the old one, the organism could be at a survival advantage over the rest of the population. The mutation is passed on to its offspring and soon the whole population has inherited the mutation. This is part of natural selection and evolution - An example of this is a mutation in bacterium that becomes resistant to antibiotics and forms a resistant strain.

Radiation and chemicals -Mutations can happen spontaneously - when a chromosome doesnt copy itself properly. However if you are exposed to Ionising radiation including x rays and UV light the chance of mutation is increased. It is also increased if you are exposed to chemicals called mutagens. If the mutations cause cancer the chemicals are often called carcinogens. Cigarette smoke contains carcinogens.

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Multiplying cells

Advantages of being multicellular - Being multicellular means you can be bigger. This means you can travel further, ingest nutrients in different ways and are less at risk of being somethings prey. It also allows for cell differentiation. Different cells can do different jobs which they have adapted to do. This means multicellular organisms can be more complex - they have specialised organs and can adapt to different environments.

Mitosis -Mitosis is when a cell reproduces itself by splitting to form two identical offspring. This happens when you want to grow, replace worn out cells or repair tissues.

The DNA in the cell is first replicated and is then coiled into double armed chromosomes. These arms are exact copies of each other.

The chromosomes line up at the centre of the cell and then divide as cell fibres pull them apart.

The two arms go to opposite poles of the cell. Membranes form around these two different sets of chromosomes.

The cytoplasm divides and you get two new cells containing the same genetic material.

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Meiosis, Gametes and Fertilisation

Meiosis creates gametes -Meiosis occurs in the testes and ovaries. Gametes are the sex cells - eggs and sperm. Body cells of mammals are diploid. This means that body cells contain two copies of each chromosome in the nucleus - one from the mother and one from the father. However gametes are haploid meaning they only have one copy of each chromosome meaning when the sperm and egg join, they form a cell with the diploid number of chromosomes.

Meiosis -The DNA first replicates and curls to form double armed chromosomes. After replication the chromosomes arrange themselves into pairs. Humans have 23 pairs. In the first division these pairs split up - the chromosomes move to opposite poles of the cell. In each of the two new cells there are no pairs - just one of each of the 23 pairs. Each new cell ends up with a mix of your mum and dads chromosomes but only half of the usual number of chromosomes. The second division is like mitosis - each chromosome splits in half and one arm ends up in each new cell. You end up with four genetically differnet cells as the process is random.

The diploid cell formed when male and female gametes combined is called a zygote. A sperms function is to carry the males DNA to the females egg. Sperm have lots of mitochondria to provide energy and an acrosome at the front of the head which releases the enzymes needed to digest the membrane of the egg cell.

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Stem cells, differentiation and growth

Animals stop growing - plants can grow continuously -In animals growth occurs due to cell division. In plants growth in height is largely due to cell enlargement - cell division only occurs in parts of the plant called meristems (at the tips of the roots and shoots).

Stem cells can turn into different cells -Cells can become specialised to do specific jobs.Most cells in the body are specialised eg white blood cells are good at fighting invaders. Some cells are undifferentiated. They can turn into tissues, organs or cells depending on what instructions they're given. Stem cells are found in early human embryos. They can turn into any cell at all. Stem cells are also found in human bone marrow. They aren't as versatile as the stem cells in the embryo as they can only turn into certain cells.

People with blood disorders can be cured by having a bone marrow transplant. The stem cells turn into new red blood cells. Scientists can extract stem cells from early human embryos for skin grafts, nerve cell treatment etc.

Some people think human embryos shouldn't be used for stem cell research as they are potential human lives.

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There are three methods for measuring growth - Length: measuring the height of a plant/animal. Easy to measure. However it doesnt tell you about changes in width/diameter, number of branches etc.      

Wet mass - Weighing the plant or animal. Easy to measure. Its very changeable - A plant will be heavier if its just rained and an animal if its just eaten/got a full bladder. Inaccurate.

Dry mass - Dry out the organism and then weigh it. Not affected by the amount of water in a plant/animal or how much its eaten. You have to kill the organism in order to use this method.

Human growth has five main phases: Infancy, Childhood, Adolescence, Adulthood, Old age. The two main phases of rapid growth occur just after birth and during adolescence. Growth stops at adulthood. 

Certain parts of the body grow faster than others. When a baby is growing in the womb, the brain grows at a greater rate than the rest of the body. This is because a well developed brain gives humans a big survival advantage.                                                                                                                                              

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Respiration is the process of releasing energy from glucose. It occurs in every cell in your body. The energy from respiration is used to make a substance called ATP which acts as the energy source for many cell processes and transports energy to where its needed in a cell. Respiration is controlled by enzymes. The rate of respiration is affected by temperature and pH.

Aerobic respiration needs oxygen -                                                                                         Glucose (C6H12O6) + Oxygen (O2) -> Carbon Dioxide (CO2) + Water (H2O) + Energy. When the respiration rate increases, so does oxygen consumption and CO2 production. This means the rate of oxygen consumption can be used to estimate metabolic rate.                                               Anaerobic respiration is without oxygen - When you exercise your body cant supply enough oxygen to your muscles for aerobic respiration. Anaerobic respiration is therefore used.It isnt as efficient as aerobic respiration as it releases much less energy per glucose molecule.              Glucose -> Lactic acid + energy                                                                                                       To repay the oxygen debt caused by anaerobic respiration (oxygen is needed to break down the lactic acid) you have to breath hard. The lactic acid has to be carried to the liver to be broken down too so your heart rate has to stay high.

RQ = Amount of CO2 produced / amount of O2 used

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Functions of the blood

Plasma is the liquid part - Plasma carries all the needed components around the body. Red blood cells, White blood cells and platelets, water, Digested food products like glucose and amino acids from the gut to body cells, Carbon dioxide from body cells to the lungs, Urea from the liver to the kidneys, Hormones and Antibodies.

Red blood cells carry oxygen -Red blood cells are small and have a biconcave shape to give them a large surface area to volume ratio for absorbing and releasing oxygen. They contain haemoglobin which is what gives blood its colour. It contains a lot of iron. Haemoglobin combines with oxygen in the lungs to become oxyhaemoglobin. In body cells the reverse happens to release oxygen to the cells,

Red blood cells dont have a nucleus freeing up more space for haemoglobin. They are very flexible meaning they can fit through the tiny capillaries.

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Blood vessels

Three types of blood vessel -Arteries (carry the blood away from the heart), Capillaries (are involved in the exchange of materials in the tissues) and Veins (carry blood to the heart).

Arteries carry blood under pressure -The heart pumps the blood at high pressure so the artery walls are thick and elastic and strong. The walls are thick in comparison to the lumen.

Capillaries are really small - Arteries branch into capillaries. They carry blood close to every cell in the body to exchange substances. They have permeable walls so substances can diffuse. The walls are only one cell thick, increasing the rate of diffusion. The lumen is very small.

Veins take blood back to the heart - The blood is at a lower pressure so the walls dont need to be as thick. They have a bigger lumen to help the blood flow. They have valves to keep the blood flowing in the right direction.

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The Heart

Double circulatory system -the first system connects the heart to the lungs. Deoxygenated blood is pumped to the lungs and then returned to the heart.The second system connects the heart to the rest of the body. Having this system means that blood can be pumped round the body at a higher temperature, increasing the rate of blood flow to the tissues, so more oxygen can be delivered to the cells.

Parts of the heart: the right atrium receives deoxygentated blood through the vena cava whilst the left atrium receives oxygenated blood through the pulmonary vein. The deoxygenated blood moves through the right ventricle which pumps it to the lungs through the pulmonary artery whilst the oxygenated blood moves through the left ventricle which pumps it round the whole body via the sorts.

Semilunar, bicuspid and tricuspid valves stop the backflow of blood.

The left ventricle has a much thicker wall than the right ventricle as it has to pump blood round the whole body rather than just to the lungs.

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Selective breeding

Selective breeding is when humans artificially select plants or animals to breed so that their genes remain in the population. Organisms are selectively bred to develop features such as maximum yield of milk, meat, grain etc, Good health and disease resistance, temperament and more. Farmers select the stock which has the best characteristics, breed them with eachother, select the best offspring and breed them togehter, and continue this process until the desireable trait gets stronger and stronger.

Reducing the gene pool is the main drawback - selective breeding reduces the number of different alleles (forms of a gene - gene pool) in a population. This is because the farmer breeds lots of animals/plants which are closely related. This is known as inbreeding. Inbreeding can cause health problems due to the reduced gene pool as the animals are more likely to have two of the same alleles. Recessive alleles are more likely to build up in an inbred population.

If a new disease appears there can be serious problems as there isn't much variation in the population meaning if one animal gains the disease, the rest of the population are likely to succumb to it.

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Genetic engineering

the idea of genetic engineering is to move genes from one organism to another to produce useful biological products.The main advantage is that you can produce organisms with useful features very quickly. However the inserted gene may have unexpected harmful effects. Genes are often inserted into bacteria to produce useful products, but if the bacteria became pathogenic the genes could make them more harmful and unpredictable.

First the gene that produces a desireable characteristic is selected. Then its cut from the DNA using enzymes and is isolated. The useful gene is inserted into the DNA of another organism. The organism then replicates and soon there is a large number of similar organisms all producing the same desireable product.

Examples of genetic engineering: Some areas of the world rely on rice for food. Vitamin A defficiency can be a problem as rice doesn't contain a lot of it. Scientists take a gene that controls beta-carotene production from carrot plants and put it into rice plants, so the beta-carotene can be changed into vitamin A. The gene from human insulin production has been put into bacteria which is cultured in a fermenter to produce more insulin. Finally, some plants have resistance to herbicides, frost damage and disease. We can cut this gene out of unuseful plants and put it into the useful ones.

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Uses and risks of cloning animals

Cloning allows you to mass produce animals with desirable characteristics, for example animals with organs suitable for transplanting into humans can be cloned so we have a constant supply of organs. Human embryos can be produced by cloning adult body cells. The embryos can then be used to provide stem cells for stem cell therapy. These cells have exactly the same genetic information as the patient, reducing the risk of rejection.Risks include that the new animals may not be as healthy as normal ones, and that cloning is a new science and may therefore have risks that we are unaware of.

Cloning humans IS possible - Ethical issues with this include;

- There would have to be lots of surrogate pregnancies,which would probably end with high rates of miscarriage and stillbirth.

- Like animal clones, human clones would proabably be unhealthy and die prematurely.

- The new clone may be physically healthy but psychologically damaged by the idea that its simply a clone of another person.

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Gene therapy and cloning animals

Gene therapy involves altering a persons genes to cure genetic disorders - there are two types of gene therapy. One involves changing the genes in body cells, specifically those which are affected by the disorder. The second type of gene therapy would involve changing the genes in the gametes. This means that every cell of the offspring will be affected by gene therapy - the offspring wont suffer from the disease. This is currently illegal, though.

Gene therapy involving gametes is controversial as it may have unexpected consequences which can cause problems in future generations. There are also fears that this kind of gene therapy could lead to the creation of 'designer babies' - when parents can choose their babies genes.

Clones are genetically identical organisms. Cloning an adult animal is done by transferring a cell nucleus. Eg) Dolly the sheep. The nucleus of a sheeps egg was removed leaving the cell without any genetic information. Another nucleus was inserted in its place. This was a diploid nucleus from an udder cell of a different sheep and had all its genetic information. The cell was given an electric shock so it started dividing by mitosis. The dividing cell (now an embryo) was implanted into the uterus of a surrogate mother sheep. The result was dolly, a clone of the sheep from which the udder cell nucleu was taken from.

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Cloning plants

Farmers take cuttings from plants and plant them to produce genetically identical copies. Cloning plants is easier than cloning animals as plant cells usually keep their ability differentiate- animal cells lose this at an early stage.

Commercial cloning involves tissue culture - The plant is chosen based on its characteristics. Then several small pieces of the plants tissue are removed. The best results come when you remove tissue from fast growing root and shoot tips. You grow the tissue in a growth medium containing nutrients and growth hormones. This is done under aseptic conditions to prevent growth of microbes. As the tissues produce shoots and roots they can be moved to potting compost to carry on growing.

Pros and cons -

- You can be fairly sure of the products characteristcs so you will only get good products and wont waste time and money growing duds.

- You can mass produce palnts that are hard to grow from seeds

- All the plants are likely to suffer from the same diseases as they have the same genes.

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